1. The Field of the Invention
The present invention relates generally to the deposition of metals on semiconductor substrates or wafers for use in the semiconductor industry. More specifically the present invention relates to methods and apparatus for more uniformly depositing such metals during a sputtering process by detecting the presence of reflected neutrals.
2. The Relevant Technology
Many deposition apparatus and techniques are known in the prior art for depositing metal film layers upon semiconductor substrates or wafers for use in the semiconductor industry. In the context of this document, the term "semiconductor substrate" is defined to mean any construction comprising semiconductive material, including but not limited to bulk semiconductive material such as a semiconductive wafer, either alone or in assemblies comprising other materials thereon, and semiconductive material layers, either alone or in assemblies comprising other materials. The term "substrate" refers to any supporting structure including but not limited to the semiconductor substrates described above. The term semiconductor substrate is contemplated to include such structures as silicon-on-insulator and silicon-on-sapphire. Of those deposition techniques, the predominant conventional techniques are evaporization, physical vapor deposition (PVD) and chemical vapor deposition (CVD). Although differing in their approaches to metal film deposition, each deposition technique must meet certain minimum criteria to be successfully employed. Since PVD processes frequently allows the conservation of the target material, improvements in step coverage and adhesion and affords better control of manufacturing parameters (i.e., deposition rates, target materials and system pressures), PVD processes are frequently favored over other existing methodologies.
One PVD process, commonly known as sputtering, takes place in a vacuum chamber where positively charged gas atoms are generated and attracted to a negatively charged target material, also in the vacuum chamber. As the ionized atoms are attracted to the target material, they accelerate, gain momentum and strike the target. This striking liberates atoms and molecules of the target material, some of which fall to rest on semiconductor substrates positioned beneath the target material near the bottom of the vacuum chamber. Thereafter, the semiconductor substrate is coated with a layer of metal which is then patterned into a circuit, for example.
One problem with sputtering is that sometimes the metal coating applied to the substrate is not uniform. Although not necessarily a problem for all types of applications utilizing this technique, in the semiconductor industry when layer thicknesses are often 5 microns or less, non-uniform layer thicknesses can sometimes cause catastrophic failures during manufacture and use of the devices on which they are fabricated. Such failures can be electrically or mechanically related. Electrical failures include electrical shorting, insufficient resistance levels and introduction of induced electrical charges. Mechanical failures include fracture because of insufficient mechanical strength and malformations such as pinholes.
Although some sputtering equipment systems are available that generally improve uniformity these systems are expensive. They not only involve high capital costs as an initial investment but also as changes, retrofits and calibrations occur. Additionally, these systems are technologically sophisticated and complicate the design processes thereof.
In general, uniformity problems are caused by various sources. One known source is neutral gas molecules that are reflected from the target material during the sputtering process. The reflected neutrals reflect off the target material and erosion tracks thereof, i.e., ruts, which are non-uniform. As a result, the growth of the metallic film is correspondingly non-uniform. The reflected neutrals also bombard the metallic film on the semiconductor substrate in a non-uniform manner which likewise adversely impacts the uniform deposition and growth of the metallic film.
Reflected neutrals are also known to be a source of re-sputtering which is also problematic to the deposited metallic film, especially when the target material is multi-compositional. For example, when the target material has more than one component, such as aluminum-copper or titanium-tungsten, the reflected neutrals during re-sputtering, again reflecting off of non-uniform erosion tracks, can potentially cause variances in the sputtering rates of the various compositions. In turn, non-uniform composition densities of the metallic film deposited on the semiconductor substrate can be observed.
Uniformity problems are even further complicated with multi-layer coatings because when a first film layer is non-uniform the next deposited layer assumes the contour of the first layer and is likewise non-uniform. This non-uniformity also continues with each additional layer deposited thereafter.
Accordingly, it is desirous to detect neutral gas molecules reflected from a target material during a sputter deposition process to, ultimately, afford improvement in the uniformity of metallic-film substrate coatings.